We paint car exhausts?
Fraid so, usually silver. At last the tubular metal ones, not the cast ones
I'm on a losing wicket here TBH as I can't prove it either way for the reasons Lance pointed out
My instinct is that tubular exhausts and stubs would have been painted black to stop them corroding after manufacture. In use, they would burn off the paint and go metallic with oxidising colours on them (not rust). If this was done in the presence of oil (which would have to be a deliberate act) that produces a relatively rust proof finish too. After long enough exposure to water, they would indeed rust.
Cast manifolds would be grey metallic when new, rusty not long afterwards
Interesting to speculate about, hard to prove
Richard